A robot cleaner may have a body, a mop, and a tank to supply water to the mop. The tank may be magnetically coupled to the body via at least one magnet and be physically coupled to the body via a groove and a protrusion The protrusion may be a bent rib extending from the tank to couple to a groove in the body, or the protrusion may be a stopper attached to a spring attached to the body and coupling with a groove in the tank. The at least one magnet may be provided at a first side with respect to a left-right center, and the protrusion and groove may be provided at a second side opposite the first side. To remove the tank from the body, the tank may be pulled near the magnet to detach the tank at the magnet, and the tank may be rotated at a point on the second side.